merge method
These styles, each taking from other's what it leaves unset.
Implementation
ButtonThemeData merge(ButtonThemeData other) => ButtonThemeData(
icon: icon?.merge(other.icon) ?? other.icon,
text: text?.merge(other.text) ?? other.text,
filled: filled?.merge(other.filled) ?? other.filled,
outlined: outlined?.merge(other.outlined) ?? other.outlined,
);